1. Field of the Invention
This invention relates to an improved process for the conversion of hydrocarbons, and more specifically for the catalytic isomerization of olefins in gasoline streams.
2. General Background
The widespread removal of lead antiknock additive from gasoline and the rising fuel-quantity demands of high-performance internal-combustion engines are increasing the need for "octane," or knock resistance, in the gasoline pool. Petroleum refiners have relied on a variety of options to upgrade the gasoline pool, including improved catalysts and processes for catalytic naphtha. The low-cost options for gasoline upgrading have been largely exploited, however, and refiners need new technology to address future gasoline-octane requirements.
Gasoline from catalytic cracking necessarily is a major target of octane-improvement efforts, as it typically amounts to 30 to 40% of the gasoline pool. Efforts to improve the cracking catalyst and process have resulted principally in increased gasoline aromatics content and reduction of low-octane components in the middle-boiling range. There is limited leverage to alter the cracking reaction to increase gasoline octane, however. The paraffin component has a higher-than-equilibrium ratio of isoparaffins to normal paraffins, and thus a higher octane than currently could be obtained by isomerization. The olefin component of the cracked gasoline has an equilibrium ratio of branched to unbranched olefins, and this can be changed only marginally in the cracking reaction.
A process for isomerizing olefins in catalytically cracked gasoline thus has considerable potential for improving the octane of the gasoline pool, but must address several problems. The process must not effect substantial isomerization of paraffins, in order to avoid changing the already-high ratio of isoparaffins to normal paraffins. The process should operate at relatively low temperature where the equilibrium ratio of branched to unbranched isomers is more favorable and by-products are minimized. An effective process also should solve the problem of highly unsaturated hydrocarbons in the feed such as acetylenes and dienes which could polymerize and foul the catalyst, thus requiring higher temperature to maintain catalyst activity and reducing catalyst life.